I've heard it said various times before that humans can't breathe pure oxygen, or at least not for very long, before it would kill us. But during a discussion on Worldbuilding, the difference between air-breathing and water-breathing came up. It made me wonder: what would happen to a fish if it had pure oxygen, rather than the ordinary gas mixture of air, dissolved in its water? Would it be safe, or have the same oxygen toxicity problems as humans?
It won't be safe. Typically there is a limit to how much oxygen can be in the water due to the physics of solubility. However, there are some circumstances where the water can be temporarily supersaturated with oxygen. For instance, near a waterfall, dam/thermal discharge, algae bloom etc.
If a fish were to remain there, they may contract gas bubble disease. Mortality of young salmon and trout has been observed after 3 days at 120% oxygen saturation.
You can read more about it at --- Fondriest Environmental, Inc. “Dissolved Oxygen.” Fundamentals of Environmental Measurements".